1. Field of the Invention
The present invention relates in general to power steering systems for automotive vehicles and more particularly to an electric motor drive-type power steering system having an electric motor for supplying a steering assist force to a steering gear for thereby multiplying a steering effort applied to the steering gear by way of a steering wheel.
2. Disclosure Information
Heretofore, the power steering systems of the hydraulic pump drive-type were mainly used. However, many of the power steering systems produced by various car makers these days are of the electric motor drive-type as disclosed in Japanese Provisional Publication No. 61-110668. For example, the power steering system is provided with sensors for detecting the steering effort applied to the steering wheel and the vehicle and supplying signals representative thereof to the controller so as to control increase and decrease of a steering assist force to be supplied from the electric motor to the steering gear in accordance with the steering effort and the vehicle speed.
The prior art electric motor drive-type power steering system is adapted to multiply the steering effort by using a single electric motor and for this reason has the following problems.
At low vehicle speed such as the case when the vehicle starts from a standstill condition (vehicle speed is zero), the reactions transferred from the road wheels to the steering gear are large so that a high output motor is necessitated for meeting the requirement of large steering assist force.
At middle to high vehicle speed, the reactions transferred from the road wheels to the steering gear are not so large, so a low output motor can be used at low to middle vehicle speed. However, in order to meet the requirement at low vehicle speed it has been the practice to utilize a high output motor also at middle to high vehicle speed. The high output motor however has a large rotational inertia and therefore has a problem of being poor in responsiveness, particularly when the direction of turning of the steering wheel is changed, that is, the direction of steering the vehicle is changed.